A frame structure of an automobile having a monocoque body is formed by joining a plurality of frame parts, produced by stamping sheet metal. A reinforcement of a lower front pillar, joined to the frame members such as an outer reinforcement of a side sill, comprises an L-shaped member having an L-shaped flat top wall, side walls extending along the edges of the top wall and flanges connected to the side wall. When such an L-shaped product is produced by stamping a blank sheet of metal, there are problems such as generation of wrinkles in the top wall and cracks in an inside flange of the L-shaped product.
If such an L-shaped product is formed by a drawing process, generation of wrinkles can be avoided. In drawing, however, it is necessary to provide a blank with a relatively large margin, which results in lower yield rate and higher production cost.
Further, for drawing processes, it is necessary to use a blank of relatively high extensibility, whereby a blank for drawing is made of a relatively low-strength material. Thus, in order to increase the collision performance of automobiles, a relatively thick blank is required, which results in an increase in the weight of the frame structure, and thus higher material cost.
Various bending methods have been proposed for producing a component having uniform cross section such as simple hat-shaped cross section, or Z-shaped cross section as described in Patent Publications 1-4. However, Patent Publications 1-4 do not disclose a method of producing a more complicated L-shaped member described above.